Powder metallurgy (PM) has been regarded as a viable and promising approach for reducing the cost of Ti fabrication because of its near-net-shape capability and the potentially melt-less nature of the process. There are generally two kinds of powder metallurgy approaches for making PM titanium products: the blended elemental (BE) method and the pre-alloyed (PA) method. The BE method, in general, refers to the pressing and sintering of blended elemental powders. Sintering is generally carried out under vacuum. The PA method refers to sintering pre-alloyed powders, which are typically produced using gas atomization or plasma rotating electrode techniques. Pre-alloyed powders have high hardness and, therefore, poor press-ability when compacted using conventional, uni-axial, cold pressing methods. Therefore, pre-alloyed powders are usually consolidated using pressure assisted consolidation techniques, such as hot isostatic pressing (HIP). Although PA products, in general, have better mechanical properties than BE products, the costs of PA products are significantly higher. Therefore, BE is still the preferred cost-effective approach.
Residual porosity, oxygen contamination, and relatively coarse microstructure after sintering limit the static and fatigue properties of BE and PA materials. One approach for reducing residual porosity is to use post-sintering, high pressure processes, such as hot isostatic pressing, which can increase the density to greater than 99.8% of the theoretical density. Any post-sintering process, however, adds extra cost to BE parts, thereby reducing the cost advantages of the BE method.
In recent years, an alternative BE technique emerged for titanium production, which is able to produce nearly pore-free BE parts directly. This technique employs vacuum sintering of hydrogenated titanium or Ti hydride (TiH2) powders instead of Ti metal powder. During sintering, TiH2 will dehydrogenate at moderate temperatures and subsequently sinter at high temperatures under vacuum. Blends of TiH2 with an appropriate ratio of 60Al-40V master alloy powder can be sintered to 98.5%-99.5% of the theoretical density in as-sintered state, in contrast to 90%-95% of the theoretical density when titanium powder was used. Although PM Ti parts produced by sintering using TiR2 powder have shown great potential, the grain sizes of as-sintered materials are usually large. The as-sintered microstructure for Ti-6Al-4V consists of coarse Widmanstätten lamellar alpha plate colony structures, which have a coarse microstructure that is not optimal with respect to tensile or fatigue strength. The as-sintered coarse microstructures can be refined only by post-sintering thermal mechanical working and heat treatments, which, once again, increase the cost of PM Ti parts, reducing the economic benefits of PM Ti.